1. Field of the Invention
The present invention relates to a method for producing a glass preform for use in the fabrication of a single mode optical fiber. More particularly, the present invention relates to a method for producing a glass preform under controlled process conditions to achieve a desired step type refractive index profile. A single mode optical fiber fabricated from the glass preform produced by the method of the present invention has a highly efficient dispersion property.
2. Description of the Related Art
One conventional method for producing a glass preform for an optical fiber is shown in FIG. 1. An oxyhydrogen flame is generated by a core-forming burner 4, and a glass-forming raw material such as SiCl.sub.4 and GeCl.sub.4 is supplied to a center part of the flame so that soot particles 6 of a glass such as SiO.sub.2 and GeO.sub.2 are synthesized by flame hydrolysis and/or oxidation reactions and deposited on a starting member which is rotated around and moved along its axis as glass soot is deposited to form a porous glass core body 1. In a similar manner, a porous glass cladding body 2 is formed by generating an oxyhydrogen flame by a cladding-forming burner 5 and supplying a glass-forming raw material such as SiCl.sub.4 to the center part of the flame so that SiO.sub.2 soot particles 7 are synthesized and deposited around the peripheral surface of the core body 1 to form a porous glass cladding body 2, whereby a porous glass preform 8 for the single mode optical fiber is produced. By dehydrating the porous glass preform 8 at a high temperature and then consolidating the dehydrated preform at a higher temperature, a transparent glass preform for the single mode optical fiber is obtained. Typically, the transparent glass preform for the single mode optical fiber produced by the above described conventional method has a refractive index profile as shown in FIG. 2.
To improve the dispersion property which is one of the transmission characteristics of the single mode optical fiber, it is desirable for the core to have a step type refractive index profile. However, as shown in FIG. 2, the conventional glass preform for the single mode optical fiber has a refractive index with an irregularity which is so-called "slope" indicated by "3" in FIG. 2.
Further, although the refractive index profile is determined during the production of the porous glass preform, there is no effective method for measuring the refractive index of the glass during the production of the porous glass preform.
One of the measures for decreasing the slope of the refractive index is described in Japanese Patent Kokai Publication No. 63-74931 (corresponding to Japanese Patent Application No. 61-216027). In this method, bulk densities of the porous glass core body and the porous glass cladding body are adjusted to desired values. Since this method achieves the desired refractive index profile by controlling the process conditions, the refractive index profile can be measured only after the porous preform is made transparent.